Cryogenic liquids may be used to cool exothermic reactions due to their refrigeration properties. A particularly direct approach would be to inject the liquid cryogen directly into the liquid reactants within a reaction vessel, thereby eliminating the potential freezing and fouling of the heat transfer surfaces. The liquid cryogen vaporizes instantaneously upon contacting the reaction mixture and transfers both the latent heat and the sensible heat to its surroundings.
However, injecting the liquid cryogen directly into the reaction vessel has proven very difficult because the liquid cryogen storage tank may be optimized only for a given pressure. For reaction tanks which are withdrawing both a liquid cryogen for cooling and a gas for blanketing, purging and other applications, for example, liquid nitrogen and nitrogen gas, it is necessary to set the tanks at the high working pressure. Therefore, the liquid cryogen must be withdrawn also at high pressure, e.g., 150 psi, but such an operating pressure is unfavorable for most liquid cryogen applications.
The problem with supplying liquid cryogen at high pressures is that the liquid cryogen will be saturated with the gaseous phase of the cryogen. When the pressure is reduced due to frictional losses, e.g., from valves or elevation, the liquid cryogen will flash off from the liquid as a gas. Since the gas may occupy several hundred times the volume of the liquid cryogen, gas will create blockage to the liquid cryogen line resulting in a substantial pressure drop and reduction in liquid cryogen supply. A reduction in the liquid cryogen supply may be a potential safety hazard when the cooling capacity is lost during the peak of self-accelerating exothermic reactions.
Conventional cryogenic cooling systems keep the supply pressure from the liquid cryogen storage tank low, e.g., about 50 psi, to minimize flashing across the valves. When there is a need for high pressure gas for blanketing, purging, etc., two separate tanks are required. For example, high pressure liquid nitrogen is transferred from trucks or the high pressure liquid nitrogen storage tank into a low pressure liquid nitrogen storage tank. The gas saturated at higher storage pressure will be venting off in the gas phase as it enters the low pressure storage tank. The vented gas is a lost product. Furthermore, flashing occurs when the liquid cryogen flows across any restricted devices such as pressure regulators or control valves. Unfortunately, these devices are necessary for precise temperature control of the reaction processes. Flashing across a control valve may cause the flow to stop due to vapor lock. Thus, it would be desirable to eliminate the need to transfer the liquid cryogen from a high pressure tank to a low pressure tank thereby reducing the lost of the vented gas. Also, it would be desirable to provide a liquid cryogen cooling system wherein flashing of the liquid cryogen is eliminated or substantially reduced.
Another conventional approach uses a sub-cooler wherein a portion of the liquid cryogen such as liquid nitrogen is diverted into a separate chamber and boiled off at reduced atmospheric pressure. The boiling liquid is heat exchanged with the main stream of liquid nitrogen at the higher pressure. Since the boiling point of the liquid nitrogen is lower at reduced pressure than the liquid nitrogen at the higher pressure, the boiling liquid nitrogen is capable of sub-cooling the liquid nitrogen at the lower pressure and condenses the nitrogen gas bubbles back into liquid. The vaporized nitrogen at the lower pressure is discharged to the atmosphere because it loses its pressure head and latent heat refrigeration value. The amount of loss will increase with the pressure of the liquid nitrogen storage tank, room temperature and frictional loss of the liquid nitrogen supply. Although the sub-cooled liquid nitrogen will have less tendency to flash off across the valves and fittings since the temperature is below its boiling point, additional liquid nitrogen must be diverted to the sub-cooler in order to provide sufficient refrigeration to sub-cool or condense the gas as the refrigeration quality of the liquid nitrogen supply is decreased. Thus, it would be desirable to eliminate the need to sub-cool the liquid nitrogen while retaining the refrigeration quality of the liquid nitrogen supply.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of and apparatus for supplying liquid cryogen at high pressure with minimal flashing.
It is another object of the present invention to provide a method and apparatus for supplying liquid cryogen at high pressure directly into a reaction vessel for optimal cooling of exothermic reactions.
A further object of the invention is to eliminate the need to sub-cool the liquid cryogen while retaining the refrigeration quality of the liquid cryogen supply.
It is yet another object of the present invention to provide a single source for both high pressure gaseous nitrogen and high quality liquid nitrogen.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.